Offshore projects in Ireland had been overlooked by many firms because of high costs and a lack of commercial finds since drilling first began in the 1970s. But prospectors such as US major Exxon Mobil and Norway's StatoilHydro now hope to strike it lucky in Ireland as access to new fields elsewhere becomes more difficult.

A study commissioned by the Irish Government said that early data showed potential reserves of 10 billion barrels of oil off Ireland's Atlantic coast. Ireland remains vulnerable to global supply disruptions because it is at the end of Europe's pipelines.

It has tried to woo oil companies with financial incentives and a recent series of offshore licensing rounds.

"The majors are looking for new places to explore and the favourable tax rate in Ireland makes it worth a punt," said Andrew Harwood, energy analyst at Wood Mackenzie.

Exploration companies pay 25% tax in Ireland, which rises to 40% for the most profitable finds – lower than the 50% rate in the UK, 78% in Norway and more than 80% levied in other regions.

The growing momentum is expected to result in offshore investment in Ireland of at least $250 million this year, according to industry body IOOA. Although low compared to else, it will be the highest level since 1978.

"High oil and gas prices, improved deepwater technology and a safe political environment means that we can expect higher drilling activity in the future," Davy analyst Job Langbroek wrote in an investor note.

While the price of oil has fallen by more than 20% from a record high of more than $147 a barrel in mid-July, prospectors are taking a longer-term view.

"The short-term nature of the spiking is not something that you could plan your exploration strategy around," said John Conroy, licence and operations manager with StatoilHydro's Irish unit, which has a number of offshore projects.

West coast

Exploration activity in Ireland has remained relatively slow, with 177 wells drilled to date, compared with about 4,000 in the UK and 1,200 in Norway. While much of the drilling has centred on the Celtic Sea off southern Ireland, no oil has been produced yet.

Gas production has totalled a modest 1.7 trillion cubic feet (tcf).

Gas from the area had supplied the bulk of Ireland's needs but fields are now nearly depleted.

Smaller Irish exploration companies, such as Providence Resources and Island Oil and Gas, aim to exploit firmer oil prices, better seismic surveying data and advances in drilling technology to develop older discoveries in the Celtic Sea and tie them to existing facilities.

The shallow water and existing infrastructure has made it easier to get production onstream much quicker.

But the biggest potential returns are seen in the frontier Atlantic margin on Ireland's west coast, characterised by harsh conditions and waters that reach more than 2,000m in places.

Drilling here is much more expensive because of the need for support vessels, with a well estimated to cost $80 million to $100 million.

"There are huge potential rewards off the west coast of Ireland, but also a higher risk," said Tony O'Reilly Jnr, chief executive of Providence Resources, which has partnered Exxon Mobil on projects in the area.

High quality

The only commercial discovery made in the area is the Corrib gas field, found in 1996 and expected to come onstream in 2009 after being beset by delays.

“There are huge potential rewards off the west coast of Ireland, but also a higher risk.”

It is hoped that Corrib, which is operated by Royal Dutch Shell with estimated reserves of about 1 tcf of gas, will replace the Celtic Sea fields and help to spur drilling in the Atlantic margin.

Patrick Shannon, a professor of geology at University College Dublin, said that while no oil discoveries had yet been made around the Atlantic margin, tests showed that any oil may lack sulphur, making it easy to refine and therefore more valuable.

"Everybody wants a Brent crude or a West Texas type crude and the Irish crude oil, certainly in the Atlantic margin, is of that quality," Shannon said. "We know there is a petroleum system but we don't yet know its extent."

This year Shell and StatoilHydro each drilled an exploration well in the Atlantic margin, the results of which are anticipated.

"It depends on near term exploration success whether people remain interested in Ireland's Atlantic acreage," said Wood Mackenzie's Harwood. "There needs to be some kind of payback."